Use the exclusion mask to improve light pollution reduction

In order to apply light pollution reduction, Nebulb must first make an estimation of the light pollution by analysing the illumination of the entire sky.

However, in many cases, apart from the existing light pollution, the sky is not completely uniform, since the celestial objects photographed (such as the galactic centre of the Milky Way or any deep space nebula) also add variations in intensity to the image.

You must take into account that, by default, Nebulb uses all the information from the sky to make the best possible light pollution estimation, so if there are large nebulae in the image (such as the galactic centre of the Milky Way) they will also be included in the estimation and, therefore, part of the information from these nebulae will also be removed when applying the light pollution reduction.

But precisely to avoid this problem, Nebulb has a special mask: the exclusion mask.

Thus, by means of the exclusion mask you can tell Nebulb which areas of the sky should not be taken into account for the estimation of light pollution, since these areas include information about nebulae or objects that you want to keep unaltered when applying light pollution reduction.

In this other article you can find more information about when and how to apply the exclusion mask: https://help.nebulb.com/portal/en/kb/articles/when-and-how-to-define-the-exclusion-mask

And below you can see an example of the difference between applying the exclusion mask and not applying the exclusion mask:

Image resulting from stacking without applying any light pollution reduction.

Result of applying light pollution reduction without having defined an exclusion mask. The galactic centre of the Milky Way is much better seen than before, but its large nebulae have become rather dull and colourless.

Process of defining and applying the exclusion mask: (1) Press the button to define the exclusion mask, (2) draw on the image the areas where there is more nebulae information (but without selecting all the Milky Way because otherwise Nebulb will have little information to estimate the light pollution) and (3) refresh the light pollution estimation.

Result of the light pollution reduction using the exclusion mask defined in the previous image. In this case the Milky Way nebulae perfectly maintain all their structure and colour, while at the same time a large light pollution reduction has been achieved.

IMPORTANT 1: When defining the exclusion mask, do not select regions that are too large, as Nebulb will then not be able to estimate the light pollution in these areas properly. Therefore, it is better to select exclusion zones of small/medium size and always leave a space between them so that Nebulb can estimate the light pollution in the entire sky. For this reason, in the example above we have not selected a large exclusion zone for the whole galactic centre, but we have split the exclusion mask into 4 smaller pieces located in the areas with more nebulae.

IMPORTANT 2: You can make as many changes as you wish to the exclusion mask to get the result you are most satisfied with. To do this, simply press the refresh masks button (3) each time you make a change to the exclusion mask.

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